The invention relates to an infrared (=IR) microscope,    with a beam path for visible light and a beam path for IR light,    wherein the beam paths are coextensive in the region of a sample position, of a Cassegrain objective, and of a first intermediate focus,    wherein the Cassegrain objective images the sample position onto the first intermediate focus,    and wherein the IR microscope is constituted in such a way as to direct IR light from the first intermediate focus to an IR detector at least in an IR viewing mode, and to image the first intermediate focus onto a flat viewing surface, in particular a flat detector surface of a camera, in the beam path of the visible light at least in an optical viewing mode.
Such an IR microscope is known from the company publication “HYPERION Series” of Bruker Optik GmbH, Ettlingen, DE, 2009.
Analytical information about a sample can be obtained by means of infrared (IR) spectroscopy. The chemical bonds in the sample absorb or reflect IR light depending on the wavelength of the IR light.
In an IR microscope, the analytical information can be obtained specifically for a certain location or a certain region of the sample. Preferably, both a beam path for visible light and a beam path for IR light are set up in an IR microscope. Because the beam path for the visible light and the beam path for the IR light are largely coextensive, the location or region of the sample from which the analytical information is to be obtained by IR light can be visualized.
To examine the sample in the IR microscope, the sample is imaged onto an intermediate focus. In the intermediate focus, a region of the sample can be selected for examination using an aperture. Imaging of the sample, which should be performed in the same manner in the beam path for visible light and in the beam path for IR light, cannot be achieved with conventional lenses because of their differing refractive properties for visible light and IR light. A Cassegrain objective is usually used to image the sample in which the visible light and the IR light are reflected on two curved (usually spherical) surfaces.
However, the Cassegrain objective results in an aberration in the imaging of the sample, namely field curvature (also known as field of curvature). The imaging of the sample is focused onto a curved surface. If the image is now detected with a flat detector surface (which is the case in all conventional video cameras), the imaging will be sharp in the center but increasingly blurred toward the outer edge.
From EP 0 051 969 A1, use of a Mangin mirror to correct field curvature in IR systems is known. Therein, the IR light is refracted on a front side (for example a germanium surface) of a Mangin mirror, reflected on a rear side, and again refracted on the front side. The Mangin mirror is integrated into a Cassegrain objective. This configuration is complicated; in particular, the Cassegrain objective is then a special component. The spectral range is also limited to the transmission range of the substrate material of the Mangin mirror.
The object of this invention is to provide an IR microscope in which the field curvature is corrected in a simple manner in the optical viewing mode, when detection is performed using a flat detector, without restricting the spectral range of the IR microscope.